1. Field of the Invention
This invention relates to film grade ethylene based hydrocarbon copolymers and film made therefrom.
2. Description of the Prior Art
U.S. Pat. No. 4,076,698 (filed originally Mar. 1, 1956 and issued Feb. 28, 1978) discloses the preparation of film grade hydrocarbon polymers made from ethylene and higher alpha olefins in slurry or solution polymerization systems. This patent teaches that, in order to produce film having optimum mechanical properties (Elmendorf Tear Strength) from ethylene copolymers, it is necessary to copolymerize the ethylene with C.sub.5 to C.sub.8 comonomers.
U.S. Pat. No. 3,645,992 (filed Feb. 15, 1968 and issued Feb. 29, 1972) teaches that within the family of partially crystalline hydrocarbon copolymers of ethylene and various higher alpha monoolefins (e.g., butene-1, hexene-1 and octene-1), the physical properties of such resins depend not only on the molecular weight and molecular weight distribution of the polymer and on the amount and type of higher alpha olefin comonomer incorporated into the copolymer, but also upon the distribution of the comonomer units along and among all the polymer molecules. A clear distinction is made between copolymers in which the comonomer units are distributed randomly along a polymer chain but do not have the same ethylene/comonomer ratio among the polymer molecules (these are termed heterogeneous copolymers) as compared to copolymers in which the comonomer units are distributed randomly along a polymer molecule and have the same ethylene/comonomer ratio among the polymer molecules (these are termed homogeneous copolymers). The patent also teaches that these homogeneous copolymers exhibit better properties in the form of extruded film than heterogeneous copolymers made with the same comonomer and having the same density.
U.S. Pat. No. 3,645,992 also teaches that homogeneous terpolymers such as those of ethylene, propylene or butene-1 and octene-1 can be prepared with the therein disclosed vanadium based Ziegler type catalyst and process. These homogeneous terpolymers had physical properties nearly equivalent to those of the corresponding homogeneous ethylene/octene-1 copolymers.
Several recently filed U.S. patent applications, i.e.,
Ser. No. 012,720, filed Feb. 16, 1979 now U.S. Pat. No. 4,302,565, by G. L. Goeke et al. and entitled "Impregnated Polymerization Catalyst, Process for Preparing and Use For Ethylene Copolymerization", and
Ser. No. 012,795 filed Feb. 16, 1979, now U.S. Pat. No. 4,243,691, by W. A. Fraser et al and entitled "Process For Molding Film From Low Density Ethylene Hydrocarbon Copolymer", disclose the preparation of ethylene hydrocarbon copolymers in a gas phase process, particularly a fluid bed process, which can be operated at relatively low pressures, of up to about 1000 psi. The copolymers are formed from a major mol percent (.gtoreq.90) of ethylene and a minor mol percent (.ltoreq.10) of one or more C.sub.3 to C.sub.8 alpha olefins. These copolymers are heterogeneous and may be used to form film therefrom.
Attempts to make film grade resins having optimum physical properties in the form of copolymers made from ethylene and hexene-1 in the gas phase fluid bed polymerization processes disclosed in these recently filed United States patent applications however, have encountered problems when C.sub.5 -C.sub.8 comonomers are employed. The polymerization reaction involved is an exothermic reaction. In order to conduct the reaction continuously in a commercially feasible manner it is necessary to recirculate the unreacted monomers and to remove excess heat (of reaction) from the recycled monomers in a heat exchanger. Where C.sub.5 to C.sub.8 comonomers are used these monomers tend to condense out in the reactor system and lead to (i) the production of sticky or tacky polymers which are difficult to handle and process, (ii) the plugging of the gas distribution plate used in such fluid bed reactors, (iii) the fouling of sensing elements inserted in the reactor system for the purpose of monitoring and controlling the reaction conditions, and (iv) reactor temperature control problems due to heat capacity variations.
C.sub.2 /C.sub.4 copolymers having a density of 0.91 to 0.94 and a melt index of about 2.0 when made with the catalysts of these recently filed U.S. patent applications (which are also the catalysts used in the present inventions) have, in compression molded film form, Elmendorf tear strength values in the range of about 10 (at 0.94 density) to about 260 (at 0.91 density). C.sub.2 /C.sub.6 copolymers having a density of 0.91 to 0.94 and a melt index of about 2.0 when made with the catalysts of these recently filed U.S. patent applications (which are also the catalysts used in the present invention) will have, in compression molded film form, Elmendorf tear strength values in the range of about 25 (at 0.94 density) to about 500 (at 0.91 density). Based on these data (and the prior art noted above) it would be expected that C.sub.2 /C.sub.4 /C.sub.6 heterogeneous terpolymers (where C.sub.4 content=C.sub.6 content) having a density of 0.91 to 0.94 and a given melt index, when made with the same catalysts, would have poor physical properties and/or Elmendorf tear strength values lying midway between the Elmendorf tear strength values of such C.sub.2 /C.sub.4 and the C.sub.2 /C.sub.6 copolymers.
It has now been unexpectedly found that copolymers formed from ethylene, and at least one of propylene and butene-1 and at least one C.sub.5 -C.sub.8 comonomer with the catalyst systems and process of the present invention, are heterogeneous copolymers, and in compression molded film form exhibit intrinsic Elmendorf tear strengths which are nearly equivalent to that of the corresponding copolymer of ethylene and one (only) of the C.sub.5 to C.sub.8 comonomer made with the same catalyst.
The heterogeneous nature of the polymers of the present invention is demonstrated by their melting points and by polymer fractionation tests.